Beam dynamics studies and test facilites
SLAC Accelerator Seminar
January 28, 2010
Erik Adli, Department of Physics, University of Oslo and CERN
The Compact Linear Collider
 The Compact Linear Collider (CLIC): study for a Multi-TeV linear collider,
with design parameters of 3 TeV COM and luminosity of 2 x 1034 cm-2s-1
 Main linac uses normal-conducting accelerating structures with record high
accelerating gradient of 100 MV/m (loaded)
 The rf break down rate of the structure field must be kept small (10-7 per
structure per pulse), the charge required to reach the luminosity goals
must be distributed within short rf pulses to fulfill rf criteria
 CLIC two-beam acceleration scheme: high-power short rf pulses for
main linacs are extracted from a high-intensity 100 A e- drive beam.
Enables good machine efficiency (~7% wall plug to beams)
100 A
drive
beam
CLIC two-beam acceleration scheme
Recommended overview article:
R. Tomas, "Overview of the
Compact Linear Collider", PAC’09,
Phys.Rev.ST-AB 13.014801
The Compact Linear Collider
Main linac params:
• frep = 50 Hz
• ttrain = 156 ns
• N = 3.7e10@2 GHz
• Pbeam = 14 MW
The CLIC decelerator
1 km
The CLIC decelerator
Objective of the drive beam decelerator:
• Produce rf power for accelerating structures, timely and uniformly
along the decelerator. Robust performance of 42 km beam line.
• Achieving a high energy extraction efficiency, to ensure good
machine wall-plug efficiency: baseline is 90% energy extraction
maximum
• Beam must be transported to the end with very small losses
• Drive Beam: 101 A, 2.4 GeV, 240 ns @ 12 GHz
1500 x 48 power extraction and transfer structures (PETS) will
convert kinetic energy to rf power along 1 km decelerator sectors.
 novel beam dynamic challenges for the decelerator
No analogue studies for the ILC – CLIC works from scratch
Base line CLIC drive beam parameters
Drive beam baseline parameters [CLIC parameters 2008] :
 E0 = 2.4 GeV
 sE / E0=1 %
 I = 101 A
 fb = 12 GHz (bunch spacing d = 25 mm)
 t = 240 ns (2900 bunches)
 Gaussian bunch, sz = 1 mm (FF = 0.97)
 eNx,y  150 mm  sx,y  0.3 mm at bmax = 3.4 mm
 Maximum power extraction allowed: hextr = 90 %
PETS parameters
 Half-aperture: a0=11.5 mm
 R'/Q = 2294 Linac-Ohm/m
 LPETS = 0.21 m
 vg = 0.45c
Resulting power power production (ss) : P  (1/4) I2 Lpets2 FF2 (R’/Q) wb / vg = 136 MW
Resulting power extraction efficiency (ss) : h = Ein/Eext = hextr FF hdist ≈ 84 %
Decelerator beam dynamics studies
Uniform power production implies that the beam must be transported to the end
with very small losses (< 1 % level). We require robust transport of the
entire beam through each of the ~1 km decelerator sectors.
PLACET simulations, including a specially designed PETS element (including
fundamental and dipole mode wake field calculations), are the main tool for
the decelerator studies.
PLACET macro-particle beam model, sliced
beam with tracking of 2nd order moments
Simulation criterion:
3s of all beam slices < ½ radius (5.8 mm)
PETS induced energy spread
hextr = 0.90
beam at decelerator end
(pilot beam, w/o beam loading compensation)
steady-state bunch at decelerator end
(pilot beam, w/o beam loading compensation)
The decelerator beam is very particular : the high group velocity PETS will induce a significant transient
part at the head of the beam, with up to 90% energy spread at the decelerator end, as well as significant
intra-bunch energy spread. To ensure reliable rf power production it is of importance that electrons of all
energies are robustly transported along the lattice. Novel beam dynamics challenges.
Beam transport and focusing strategy
• Focusing strategy: lowest energy particles ideally see constant phase-advance m90
• Higher energy particles see phase-advance varying from m90 to m10 (towards the end of
the lattice)
• Perfect machine and beam : high energy envelope contain in low energy envelope
• Energy acceptance (e.g. of generated halo particles) only -3% of E0 at the entrance; but increasing along the lattice
• Implies that each of the (~ 40'000) quadrupoles should ideally have a different gradient
Least decelerated and most decelerated particle along the lattice :
• least dec. has a tune of m≈70, and an increase of b of
3-s
bFmax(E0)/bF0 = 4.4
envelope for
• most dec. has a tune of m≈135, and an increase of action of
perfect
machine:
Jmax(E0) / J0 = g0/gf = 10
PETS: effect on drive beam
Strong PETS higher-order of the modes combined with the very high drive beam
current : strong wake field effects, which must be studies carefully.
Reminder: PETS design (from Igor Syratchev, CERN) :
Full simulation of PETS HOM
fields, for a single driving
bunch. Courtesy of A. Candel
(SLAC).
Transverse wake calculations
 Fundamental mode longitudinal field builds up and provides rf power. Transverse
integrated field, induced by beam dipole moment, will deflect particles.
Ultra-relativistic approximation: average integrated
transverse force, evaluated in a frame moving with the
particle: wake function per unit length, W'T(z). Dipole
wake: force ~ offset of driving particle.
PLACET PETS elements: time-domain
implementation allows to implement an
artbitrary number of high-group velocity
HOM modes, to regenerate the calculated
HOM spectrum from rf simulations.
Transverse impedance spectrum : rf simulation and reconstruction
Beam dynamics challenge: dipole wake
Sufficient mitigation of transverse electromagnetic forces, due to the PETS dipole
wake, has been a major challenge for the
two-beam accelerator concept.
The natural amplification due to the dipole
wake is large, but the PETS induced energy
spread mitigates the amplification.
However, this constrains however the PETS
design:
Principal effect of dipole wake: resonant linear
increase of betatron amplitude of driven particle.
Rf power production is proportional with (R'/Q) / vg = const.
However, PETS with too low group velocity do not develop
energy spread fast enough to decohere the wake build-up
Decoherence due to intra-bunch energy spread,
with respect to point-like bunches
Dipole wake: PETS baseline design reached
Large series of potential PETS design variants have been examined for
robust mitigation of the transverse wake, for all beam modes, and
injection errors. After thorough cooperation rf experts and EA, a PETS
baseline indicating adequate wake mitigation, has been secured.
[I. Syratchev, D. Schulte, EA and M. Taborelli, "High
RF Power Production for CLIC", Proceedings of
the 22nd IEEE Particle Accelerator Conference,
2007]
Beam dynamics challenge: orbit correction

Second challenge is the effect of machine misalignment. In particular: kicks from misaligned quadrupoles
might drive beam envelope far out of vacuum chamber, even for very tight pre-alignment of 20 mm.
Estimate for uncorrected machine sets scale :
With 1000 quadrupoles and 20 mm rms offset, the
expected centroid envelope is ca. 4 mm.
90% energy spread of decelerator beam
poses a challenge for beam transport :
Dispersive trajectories of higher / lower
energy particles : 1-to-1 correction does only
properly correct the beam centroid properly.
Beam transport for ideal injection into a misaligned machine
Beam transport for ideal inj. into a 1-to-1 corrected machine
Dispersion-free steering
We seek to improve the situation by imposing that
particles of different energy shall all follow same
trajectory – i.e. minimizing the energy dependence
of the trajectories.
We propose a scheme based on drive beam bunchmanipulation and exploiting PETS beam
[EA and D. Schulte,
loading, to generate a test-beam.
“Beam-Based Alignment for the CLIC Decelerator”, EPAC’08]
Energy profile of main beam and example test-beam
Beam transport for ideal inj. into a dispersion-free steered machine
Performance of beam-based alignment
 Results of simulation including the combined effects of wake
fields and misalignment for base line parameters :
 squad = sBPM = 20 mm, 1 mrad
 sBPM,res = 2 mm
 nominal GdfidL wake level (Q=Q0)
 quadrupole mover step size 1 mm
3-s envelope of 500 simulated machines (worst case)
Ultimate goal of beam dynamics studies:
pin-point component specification
Lattice component specifications are driven by wake
mitigation and correction strategies
Need tight focusing for sufficient wake mitigation.
- Baseline: one quadrupole per meter (<b> = 1.25 m)
Need sufficent component alignment precision for initial
correction.
- Baseline: BPM and quadrupole alignment of 20 um
Need sufficient BPM precision for dispersion-free steering
performance
- Baseline: BPM precision of 2 um
Specifications
Tolerance
Value
Comment
PETS offset
100 mm
rc < 1 mm fulfilled
PETS angles
~ 1 mrad
rc < 1 mm fulfilled
Quad angles
~ 1 mrad
rc < 1 mm fulfilled
Quad offset
20 mm
Must be small to be able to
transport alignment beam
BPM accuracy
20 mm
Must be small to be able to
perform initial correction
~ 2 mm
Allows efficient suppression
envelope growth due to
dispersive trajectories
(incl. static misalignment
and elec. error)
BPM precision
(diff. measurement)
Static tolerances
Tolerance
Value
Comment
Quadrupole
position jitter
1 mm
r/r0 < 5 %
Quadrupole
field ripple
1 10-3
r/r0 < 5 %
Current jitter
< 1%
Stability req. only –
RF power constraints
might be tighter.
Beta mismatch,
db/b
~10 %
r/r0 < 5 %
Dynamic tolerances
Beam envelopes for decelerator baseline, 1-to-1 correction, and dispersion-free steering – worst of 500 machines
CLIC Test Facility 3
The CLIC Test Facility 3 is designed to demonstrate the CLIC two-beam
acceleration scheme and drive beam generation (12 GHz 28 A pulse),
including :
- fully loaded linac ( > 90% energy transfer demonstrated)
- bunch combination (2 x 4 combination, total of 26 A, demonstrated)
- CLIC experimental area with test of two-beam acceleration and deceleration
[S. Bettoni et al., "Achievements in
CTF3 and Commissioning Status",
PAC'09, 2009]
Decelerator test-facilities
Decelerator sector: ~ 1 km, 90% of energy extracted
Two-beam Test Stand:
test the characteristics of
a single PETS
Test Beam Line:
test of beam transport
where a large fraction of the
energy is extracted, under
betatron motion (16 PETS)
CLEX
Two-beam Test Stand PETS experiments
 The first 12 GHz CLIC PETS prototype (2008) is tested with beam in the Two-beam
Test Stand CLIC Test Facility 3 at CERN :
CLIC Experimental Area
The CTF3 Two-beam Test Stand experiment
(University of Uppsala, Sweden)
I. Syratchev, EA et al.,"High-Power Testing of X-Band CLIC
Power Generating Structures" PAC'09, 2009
CLIC 12 GHz PETS prototype
TBTS: experimental set-up and field recirculation
 To facilitate large power production with available CTF3 current, the Two-
beam Test Stand produced rf field is recirculated back into the PETS :
The Two-beam Test
Stand in its "PETS
only " configuration
The formulae for PETS field, power and voltage have
been extended to include recirculation :
The Two-beam Test Stand is equipped
with four rf windows (diode and I/Q)
and five inductive BPMs, including one
in a spectrometer line. Allows for
precision measurement and correlation
of both the electron beam and the rf
power.
TBTS: experimental results


Using the developed theoretical framework,
the data from the 2008 Two-beam Test
Stand has been analysed, in particular rf
power, rf phase and beam energy loss
has been measured and compared with
theory expectations.
Statistics: rms disagreement lies within
10% for 75% quartile for power
reconstructions and within 20% for energy
reconstructions. The analytical model,
derived from basic principles, yield a good
agreement and understanding of the
physics of resonant power production with
recirculation, and a basis for detecting
break down.
[EA et al. "First Beam Tests of the CLIC
Power Extraction Structure in the Two-beam Test Stand",
proceedings of DIPAC'2009].
TBTS break down
Example of pulses with observed pulse shortening. Precise measurement of rf amplitude
and phase will be used to bunchmark rf breakdown physics (work in progress)
Test Beam Line
Test Beam Line: Transport of the 28 A CTF3 Drive Beam, while extracting more than 50%
of the energy using 16 PETS, each producing CLIC level rf power, with small loss level.
Optimized segmented dump for complete energy
measurement
Quadrupole movers (CIEMAT)
Precision inductive BPMs (IFIC ES.)
Loss monitors (Cockroft I.)
Test Beam Line – End of 2009
Parameters CLIC versus TBL
Test Beam Line: experiments
3
<-
1, 2
->
• Precision correlation of 1) expected energy extraction (from intensity and bunch form), 2)
rf power measurement and 3) dump energy measurements. Precision correlations
(aiming for ~ 5 %) will show
- that we fully understand and can operate the drive beam rf power generation
- that neither wakes nor energy spread impede transport (loss monitoring)
- performance tests of first series of 12 GHz PETS and couplers
• Test of the proposed decelerator orbit correction-schemes,
using bunch manipulation and exploitation of the beam loading
-proof of principle demonstration for decelerator correction
-performance tests of BPMs and quadrupole movers
• Potential verification of resonant wake build-up
(might require resonant kickers/BPMs)
• Benchmarking of drive beam / PETS part of
the PLACET code
CTF3 linac: Dispersion-free steering
• As a test-case for the Dispersion-Free Steering, was applied to the CTF3 fully loaded linac :
• Test-case with large simulated BPM offsets was defined :
CTF3 linac structures
• Steering close to real center trajectory instead of (artifically) offset BPM centers
• in practice: DFS indicate where problems are located
[EA et al., “Status of an Automatic Beam Steering for
• Disperison reduced by a factor 3 with respect to 1-to-1 the CLIC Test Facility 3”, Proceedings of Linac’08]
Conclusions
Decelerator: novel beam dynamics challenges due to
the 90% energy extraction
42 km of beam line as the rf power source: must ensure
robust beam transport
Simulation studies show satisfactory performance for
the decelerator, following from
- sufficient mitigation of PETS wakes
- dispersion-free correction scheme
- tight, but feasible, component tolerances
Important issues still remain, but are being adressed:
- experiment tests of beam transport with large energy extraction
- machine protection issues related to the 100 A beam
- vacuum and other technical considerations
For more details about the CLIC decelerator: see EA, "A Study of the Beam Physics
in the CLIC Decelerator", Ph.D. thesis, University of Oslo (2009)
http://eadli.home.cern.ch/eadli/clic_decelerator_thesis.pdf
Thank you for your attention
TBTS break down
Effect on reducing number of BPMs
N=1
N=2
N=3
N=4
(perfect BPMs and single machine simulated, for illustration purposes)
PETS: effect of inhibition

"Petsonov": on/off mechanism

Simulated as R/Q=0, QT=2QT0 (worst-case)
 Effect of inhibition on the beam dynamics:
the lack of deceleration leads to higher minimum beam energy and
thus less adiabatic undamping and less energy spread
 dipole wake kicks increase; for a steered trajectory the change of kicks
will in addition spoil the steering
 the coherence of the beam energy will increase, and thus also the
coherent build up of transverse wakes

A number of random PETS
inhibited (averaged over
100 seeds)
Negligible effect on beam envelope for up to 1/3 of all PETS inhibited, and even more for a DFS steered machine
PETS: estimation of accepted break down voltage
 Maximum accepted transverse voltage accepted if we
require rc < 1 mm due to this kick
Quadrupole jitter and failure
Losses as function of
random quad failure
Envelope increase as
function of quad jitter
Lattice focusing
For a given optics 3% change,
or more, in initial current or
energy will induce losses
TBL energy extraction
Two-beam Test Stand
TBTS: the primary test-bed for single PETS performance
Particular interest for the decelerator studies: verification
of transverse wake :
- measurement of beam deflection, TBTS kick angle
measurement precision of 10 urad (expected kick; few 10
urad/mm centroid offset (5 A) ) – first benchmarking of
PETS code
- direct measurement of transverse wake with rf antennas
On-going work
Illustration of two-beam acceleration
Animation provided by A. Candel, Stanford Linear Accelerator Centre, using code T3P